Dark Arts was a class taught at several wizarding schools. Unforgivable Curses and numerous other forms of Dark magic were taught in this class. It is possible that students were used as victims as a form of punishment.

Hogwarts School of Witchcraft and Wizardry

Salazar Slytherin

Salazar Slytherin may have wanted to teach his students spells of which the other three founders disapproved, and it is a possibility that he had initially constructed the Chamber of Secrets as a place in which to teach the Dark Arts.[1]

1997-1998 school year

"Amycus, the bloke, he teaches what used to be Defence Against the Dark Arts, except now it's just the Dark Arts. We're supposed to practise the Cruciatus Curse on people who've earned detentions..."